Calories burned from weight lifting

TL;DR to directly address your question; I'd say no more than 200 calories per 30 min session. Start there and adjust accordingly.

Reasoning: 6'1" 172 lbs 28 y/o male here (for reference). I've not had good luck with a variety of HR monitors for tracking lifting either. I workout 75-90 minutes 3x/week and have back-calculated my maintenance TDEE since last July to average out to 2450. Desk job so I'd call it sedentary outside of intentional exercise, which most calculators spit out ~2200 cal before exercise. Since I only lift 3 days per week and the TDEE is assessed over the weekly average, the workout days would average 2783.333 to result in a weekly average of 2450, or 583 workout calories. For comparison, my Polar H7 heart rate monitor would easily give me 700+ per workout. I do a full body split about with about a 50/50 focus between strength and hypertrophy rep ranges (i.e. heavy squats and deadlifts 3 x 1-3 and pullups, bench, rows, presses 4 x 8+). Easy math if you round 583 to 600 and divide by 3 for the time, hence the guess of 200 calories per 30 min session.

My HRM calculates the same ballpark burn (3-5 calories per minute) as the default MFP "strength training" entry (3 per minute).

http://calorielab.com/burned/ This link will give you some estimates based on type of training and your weight. Rough but at least it is a jumping off point.